The present invention relates to a phase change type optical recording medium.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting. One typical rewritable optical recording medium is of the phase change type wherein the recording layer is irradiated with a laser beam to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reading. Optical recording media of the phase change type are of great interest since the optical system of the driving unit used for their operation is simpler than that for magneto-optical recording media.
Most optical recording media of the phase change type use Ge--Sb--Te base or chalcogenide materials which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
When information is recorded on a phase change type optical recording medium, the recording layer is irradiated with a laser beam of power (recording power) high enough to bring the recording layer to a temperature higher than the melting point thereof. The recording layer is melted at spots with the recording power applied thereon, and then quickly cooled so that recorded marks of amorphous nature can be formed. When the recorded marks are erased, on the other hand, the recording layer is irradiated with a laser beam having such a relatively low power (erase power) as to bring the recording layer to a temperature higher than the crystallization temperature thereof but lower than the melting point thereof. The recorded marks with the erase power applied thereon go back to the amorphous state because they are slowly cooled down after heated to the temperature higher than their crystallization temperature. With the phase change type optical recording medium, it is thus possible to modulate the intensity of a single light beam for overwriting.
With a phase change type optical recording medium comprising a recording layer constructed of a Ge--Sb--Te base material and dielectric layers provided on both sides thereof, each composed mainly of ZnS, however, the number of overwritable cycles is limited to several thousand cycles or so because of increased jitters and bit error rates due to repetition of overwriting. A possible leading reason for the increased jitters and bit error rates due to repetition of overwriting could be a composition change of the recording layer caused as by the diffusion of elements between the recording layer and the dielectric layers adjacent thereto.
For instance, JP-A 2-64937 discloses one approach to improving the number of overwritable cycles. The publication describes that repetitive recording performance can be improved by providing one or both sides of a recording layer with a heat-resistance protective layer or layers, and providing at least one of the heat-resistant protective layer with a protective layer having a volume modulus smaller than that of the heat-resistant protective layer. The publication refers to MoS.sub.2, ZnS, ZnSe, etc. as the material that forms the protective layer having a small volume modulus, and to silicon carbide, silicon nitride, and aluminum oxide as the material that forms the heat-resistant protective layer. Further, Example 2 therein teaches that SiN.sub.x layers, each of 20 nm in thickness, are formed on both sides of a recording layer of 100 nm in thickness, and the recording layer is provided on its laser beam-striking side with ZnS at a thickness of 100 nm and on the opposite side with ZnS at a thickness of 200 nm. The publication then alleges that this arrangement shows repetitive recording performance high enough to ensure scores of thousands recording cycles.
Referring here to a prior art phase change type optical recording medium, it has a general structure wherein a recording layer sandwiched between dielectric layers is provided on a substrate, and a metal reflective layer is provided on the upper dielectric layer. In the medium of this structure, however, the absorptance of the recording layer is generally higher in an amorphous state than in a crystallographic state. A problem with this is that the size of recorded marks formed upon overwriting is distorted under the influences of the crystallographic state before recording, resulting in an increase in the jitters.
To reduce such jitters, JP-A 8-124218 proposes an optical information recording medium comprising a substrate and a multilayer structure provided thereon, which comprises, in order from the substrate, a first dielectric layer, a recording layer, a second dielectric layer, a reflective layer, a third dielectric layer and an ultraviolet-curing resin layer, wherein:
Ac&gt;Aa where Ac is a light absorption of said recording layer in a crystalline region and Aa is a light absorption of said recording layer in an amorphous region,
a transmitting, extremely thin metal film, Si or Ge is used for said reflective layer, and
a dielectric material having a refractive index of greater than 1.5 is used for said third dielectric layer. By the provision of the light-transmitting reflective layer and the third dielectric layer having a high refractive index, it is possible that Ac&gt;Aa while the reflectance difference between the crystallographic state and the amorphous state is kept large. In the present disclosure, a structure where the difference between Ac and Aa is small or Ac&gt;Aa will be called an absorption coefficient control structure.
Even with the absorption coefficient control structure, however, the jitters are found to increase due to repetitive overwriting.
One object of the invention is to provide a phase change type optical recording medium using a recording material based on a Ge--Sb--Te or other system, wherein the number of overwritable cycles is significantly increased.